how do i know im ovulating? !?!?!?

Last friday (august 11th) I had this when I was wiping. I began my period July 21st ended the 24th. the 31st of july I had sex and he came in me three times. lastnight he came in me three times. I took pregnancy tests and they all came back negative. I would also like to add that I got the nexplanon removed june 1st after having it for two years. during those two years I had NO period. black fuzzies are from my undies